Steel and metal products and machine components are commonly plated or coated for corrosion protection, hardness, friction reduction and appearance. Platings such as chromium, nickel, copper, gold, etc. are commonly applied by an electroplating dipping process. For close tolerance and wear surface applications, these processes are difficult to tightly control to achieve uniform coatings which have the desired physical properties such as hardness, durability and resistance to cracking.
Modern metal coating processes such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) provide improved coating uniformity, strength and hardness. These types of metal coating processes are widely applied to cutting tools and machine components which bear heavy mechanical loads and are in moving contact with other steel or metal components. Metal coatings are applied to steel or metal parts to withstand contact with or cutting of other steel or metal parts. These types of coatings, having extreme hardness and strength, have to the inventors' knowledge not been used in applications where mechnical contact is made with softer non-metallic materials.
Machine components which are in moving contact with components or parts made of non-ferrous/non-metallic materials have also been plated, such as chrome plating of plunger and housing components of reciprocating pumps, which slide against packings made of leather, plastic, rubber or other materials. A common failure of this type of arrangement occurs when cracks form in the chromium plating layer. The cracks form due to volume contraction which occurs when the as-deposited chromium hydrides decompose to molecular hydrogen and chromium metal during post-plating bake-out. Post plating grinding can also produce foreign chrome particles which can damage a seal. Eventually small particles of the chromium layer imbed in the packings. The packings then act as a tool holder of the particles which cut into and score the plated component as it continues to slide against the packings, forming abrasions in the plated surface. The scored abrasions on the plated surface in turn damage the packings, ultimately causing the seal between the packings and the plated surface to fail. Also, as the packings are damaged, they are more likely to collect oxide particles, such as titanium dioxide, from fluid material such as paint being transferred through the pump. These particles can have hardness comparable to chrome and further contribute to scoring of the plunger. Failure of the seals in a reciprocating style pump causes loss of output pressure and loss of containment of the pumped fluid and contamination of other pump components.
In analyzing these type of seal failures, it is most intuitive to suspect the relatively softer non-metallic material, of which the packings are made, as the failing component. However, the inventors have discovered that the above described failure process starts with the failure of the plating or coating of the steel or metallic parts which bear against the packings. FIGS. 3-6 are micrographs of a chrome plated surface of a reciprocating pump plunger component designed for sliding/sealing contact with a non-metallic packing such as V-rings made of Teflon. In FIGS. 3 and 4, chrome particles are indicated at CP, and linear scores S are clearly seen running in the direction of reciprocation of the part past a seal. FIG. 5 shows an unused surface of a pump plunger chrome plated to prior art design specifications. Cracks CC are shown in the coating, along with chrome particles CP. FIG. 6 shows a plunger surface after 500 cycles of operation in sliding contact with a packing, exhibiting cracks CC, chrome particles CP within the cracks, and two score lines S.
FIG. 7 shows a mixture of particles P, including chromium and titanium dioxide, imbedded in a Teflon V-ring used as a packing in a reciprocating pump. Linear score marks S are visible in the glass fibers F embedded in the Teflon ring. FIGS. 8 and 9 show chromium particles CP embedded in a Teflon packing ring. And FIG. 10 shows a titanium dioxide particle P from paint trapped in a score S in a glass fiber in a V-ring of a pump packing. These micrographs support the inventive discovery that a source of seal failure in combined machine components of metallic and non-metallic materials is the metallic component such as the plating layer on a reciprocating plunger.
In testing, a reciprocating pump in which the plunger is chrome plated to design specifications and in accordance with quality controls, seal failures have occurred at as few as 100,000 pump cycles. The thickness of chrome plating, on the order of approximately 0.004-0.008 inches, requires substantial pre-grinding of the plunger stock to arrive at post-plating tolerances. A post plating grind is also required. Both grinding operations add significantly to manufacturing costs.